He added that the conservative candidate "could further rip France's social fabric apart", but that he thought Ms Royal had "done rather well" in the debate.

He did not say whether he would back the socialist challenger or abstain, adding he would be unlikely to make a statement before the second round.

Last week, Mr Bayrou said at a press conference he could not back either candidate as their current policies would not be good for the country.

Mr Sarkozy and Ms Royal were vying for votes from the 18% of voters who backed Mr Bayrou - who came third in the first round held on 22 April - as well as those who supported far-right nationalist Jean-Marie Le Pen.

Heated debate

Mr Sarkozy has said he thought the debate was dignified, though he found Ms Royal too combative.

"I was a bit astonished at times by a certain aggressiveness in Madame Royal," he told French radio.

Ms Royal, who during the debate accused Mr Sarkozy of "political immorality", defended her tough style.

"You can never go too much on the offensive when it comes to defending convictions and values," she said.

The most heated exchange came during the second half of the debate as the contenders discussed school places for children with disabilities.

Ms Royal accused Mr Sarkozy of "political immorality" for dismantling socialist measures on the issue.

Mr Sarkozy attacked his opponent for losing her temper - a criticism often levelled at Mr Sarkozy himself.

The rivals also debated public sector reform and clashed over employment. Mr Sarkozy said the 35-hour week, which was brought in by the socialists, had been a disaster for the economy. He said France needed to work more.

He also criticised Ms Royal's pension policy as vague.

Mr Sarkozy won 31.2% of the votes and Ms Royal won 25.9% in the first round of the election.